diff --git a/README.md b/README.md
index 6a640cf..ebb02ba 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,7 @@
 <!-- Footprint pad geometry in the catalog is fetched from EasyEDA/LCSC per LCSC code
      (packageDetail.dataStr PAD records, 1 unit = 10 mil = 0.254 mm) — not derived from
-     package names. See the catalog_pads_merge function. -->
+     package names. See the catalog_pads_merge function. Coordinates are mm, origin at
+     the package origin, +Y down (SVG's direction), so they render top-down as-is. -->
 # parts-store
 
 PromptPCB Parts — open curated component store (catalog, PBR 3D viewer, submit-with-LLM)
\ No newline at end of file